Magnetic sensing is used in a variety of applications. In particular, in geological surveys there is a need to measure magnetic fields over distance, in order to help prospectors searching for minerals, or in particular, oil and gas deposits.
Currently, individual magnetic detectors are lowered into bore holes to give data at different depths. These detectors are very expensive, and each one needs separate connection to the surface where data capture and monitoring equipment is based. Also, the environment in these bore holes, or “down-holes” is very demanding and may not be suitable for any fine instrument.
Typically such detectors operate by inserting a fixed magnet into another down-hole at some distance from the down-hole containing the detectors. By detecting the magnetic field from the magnet at the detectors, information on the geology in the rocks, the susceptibility and magnetic nature of the minerals between the two down-holes can be detected.
As these holes can be many miles deep, in order to yield a finely structured tomography, it is necessary to have many detectors at short intervals down the holes. However due to the high price of detectors and difficult down-hole conditions, it is common for a very small number of detectors to be used, or for just one detector to be lowered the whole distance.
If is therefore desirable to provide a magnetic field detector that is capable of detecting a magnetic fields over a long distance.
It is further desirable to provide a linear detector which gives continuous data along its whole length, thereby speeding up the survey and giving more reliable data that is not subject to time based variations in temperature and pressure, for instance to create a 3D map or tomography of the geological strata being investigated.
The ability to measure magnetic fields over long distances or in environments where precision instruments may not easily be used would also be desirable for instance in threat detection in maritime applications.